1SSH(1) BSD General Commands Manual SSH(1)
2
4 ssh — OpenSSH remote login client
5
7 ssh [-46AaCfGgKkMNnqsTtVvXxYy] [-B bind_interface] [-b bind_address]
8 [-c cipher_spec] [-D [bind_address:]port] [-E log_file]
9 [-e escape_char] [-F configfile] [-I pkcs11] [-i identity_file]
10 [-J destination] [-L address] [-l login_name] [-m mac_spec]
11 [-O ctl_cmd] [-o option] [-p port] [-Q query_option] [-R address]
12 [-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]] destination
13 [command [argument ...]]
14
16 ssh (SSH client) is a program for logging into a remote machine and for
17 executing commands on a remote machine. It is intended to provide secure
18 encrypted communications between two untrusted hosts over an insecure
19 network. X11 connections, arbitrary TCP ports and UNIX-domain sockets
20 can also be forwarded over the secure channel.
21 ssh connects and logs into the specified destination, which may be speci‐
23 fied as either [user@]hostname or a URI of the form
24 ssh://[user@]hostname[:port]. The user must prove their identity to the
25 remote machine using one of several methods (see below).
26 If a command is specified, it will be executed on the remote host instead
28 of a login shell. A complete command line may be specified as command,
29 or it may have additional arguments. If supplied, the arguments will be
30 appended to the command, separated by spaces, before it is sent to the
31 server to be executed.
32 The options are as follows:
34 -4 Forces ssh to use IPv4 addresses only.
36 -6 Forces ssh to use IPv6 addresses only.
38 -A Enables forwarding of connections from an authentication agent
40 such as ssh-agent(1). This can also be specified on a per-host
41 basis in a configuration file.
42 Agent forwarding should be enabled with caution. Users with the
44 ability to bypass file permissions on the remote host (for the
45 agent's UNIX-domain socket) can access the local agent through
46 the forwarded connection. An attacker cannot obtain key material
47 from the agent, however they can perform operations on the keys
48 that enable them to authenticate using the identities loaded into
49 the agent. A safer alternative may be to use a jump host (see
50 -J).
51 -a Disables forwarding of the authentication agent connection.
53 -B bind_interface
55 Bind to the address of bind_interface before attempting to con‐
56 nect to the destination host. This is only useful on systems
57 with more than one address.
58 -b bind_address
60 Use bind_address on the local machine as the source address of
61 the connection. Only useful on systems with more than one ad‐
62 dress.
63 -C Requests compression of all data (including stdin, stdout,
65 stderr, and data for forwarded X11, TCP and UNIX-domain connec‐
66 tions). The compression algorithm is the same used by gzip(1).
67 Compression is desirable on modem lines and other slow connec‐
68 tions, but will only slow down things on fast networks. The de‐
69 fault value can be set on a host-by-host basis in the configura‐
70 tion files; see the Compression option.
71 -c cipher_spec
73 Selects the cipher specification for encrypting the session.
74 cipher_spec is a comma-separated list of ciphers listed in order
75 of preference. See the Ciphers keyword in ssh_config(5) for more
76 information.
77 -D [bind_address:]port
79 Specifies a local “dynamic” application-level port forwarding.
80 This works by allocating a socket to listen to port on the local
81 side, optionally bound to the specified bind_address. Whenever a
82 connection is made to this port, the connection is forwarded over
83 the secure channel, and the application protocol is then used to
84 determine where to connect to from the remote machine. Currently
85 the SOCKS4 and SOCKS5 protocols are supported, and ssh will act
86 as a SOCKS server. Only root can forward privileged ports. Dy‐
87 namic port forwardings can also be specified in the configuration
88 file.
89 IPv6 addresses can be specified by enclosing the address in
91 square brackets. Only the superuser can forward privileged
92 ports. By default, the local port is bound in accordance with
93 the GatewayPorts setting. However, an explicit bind_address may
94 be used to bind the connection to a specific address. The
95 bind_address of “localhost” indicates that the listening port be
96 bound for local use only, while an empty address or ‘*’ indicates
97 that the port should be available from all interfaces.
98 -E log_file
100 Append debug logs to log_file instead of standard error.
101 -e escape_char
103 Sets the escape character for sessions with a pty (default: ‘~’).
104 The escape character is only recognized at the beginning of a
105 line. The escape character followed by a dot (‘.’) closes the
106 connection; followed by control-Z suspends the connection; and
107 followed by itself sends the escape character once. Setting the
108 character to “none” disables any escapes and makes the session
109 fully transparent.
110 -F configfile
112 Specifies an alternative per-user configuration file. If a con‐
113 figuration file is given on the command line, the system-wide
114 configuration file (/etc/ssh/ssh_config) will be ignored. The
115 default for the per-user configuration file is ~/.ssh/config. If
116 set to “none”, no configuration files will be read.
117 -f Requests ssh to go to background just before command execution.
119 This is useful if ssh is going to ask for passwords or
120 passphrases, but the user wants it in the background. This im‐
121 plies -n. The recommended way to start X11 programs at a remote
122 site is with something like ssh -f host xterm.
123 If the ExitOnForwardFailure configuration option is set to “yes”,
125 then a client started with -f will wait for all remote port for‐
126 wards to be successfully established before placing itself in the
127 background. Refer to the description of ForkAfterAuthentication
128 in ssh_config(5) for details.
129 -G Causes ssh to print its configuration after evaluating Host and
131 Match blocks and exit.
132 -g Allows remote hosts to connect to local forwarded ports. If used
134 on a multiplexed connection, then this option must be specified
135 on the master process.
136 -I pkcs11
138 Specify the PKCS#11 shared library ssh should use to communicate
139 with a PKCS#11 token providing keys for user authentication.
140 -i identity_file
142 Selects a file from which the identity (private key) for public
143 key authentication is read. You can also specify a public key
144 file to use the corresponding private key that is loaded in
145 ssh-agent(1) when the private key file is not present locally.
146 The default is ~/.ssh/id_dsa, ~/.ssh/id_ecdsa,
147 ~/.ssh/id_ecdsa_sk, ~/.ssh/id_ed25519, ~/.ssh/id_ed25519_sk and
148 ~/.ssh/id_rsa. Identity files may also be specified on a per-
149 host basis in the configuration file. It is possible to have
150 multiple -i options (and multiple identities specified in config‐
151 uration files). If no certificates have been explicitly speci‐
152 fied by the CertificateFile directive, ssh will also try to load
153 certificate information from the filename obtained by appending
154 -cert.pub to identity filenames.
155 -J destination
157 Connect to the target host by first making a ssh connection to
158 the jump host described by destination and then establishing a
159 TCP forwarding to the ultimate destination from there. Multiple
160 jump hops may be specified separated by comma characters. This
161 is a shortcut to specify a ProxyJump configuration directive.
162 Note that configuration directives supplied on the command-line
163 generally apply to the destination host and not any specified
164 jump hosts. Use ~/.ssh/config to specify configuration for jump
165 hosts.
166 -K Enables GSSAPI-based authentication and forwarding (delegation)
168 of GSSAPI credentials to the server.
169 -k Disables forwarding (delegation) of GSSAPI credentials to the
171 server.
172 -L [bind_address:]port:host:hostport
174 -L [bind_address:]port:remote_socket
175 -L local_socket:host:hostport
176 -L local_socket:remote_socket
177 Specifies that connections to the given TCP port or Unix socket
178 on the local (client) host are to be forwarded to the given host
179 and port, or Unix socket, on the remote side. This works by al‐
180 locating a socket to listen to either a TCP port on the local
181 side, optionally bound to the specified bind_address, or to a
182 Unix socket. Whenever a connection is made to the local port or
183 socket, the connection is forwarded over the secure channel, and
184 a connection is made to either host port hostport, or the Unix
185 socket remote_socket, from the remote machine.
186 Port forwardings can also be specified in the configuration file.
188 Only the superuser can forward privileged ports. IPv6 addresses
189 can be specified by enclosing the address in square brackets.
190 By default, the local port is bound in accordance with the
192 GatewayPorts setting. However, an explicit bind_address may be
193 used to bind the connection to a specific address. The
194 bind_address of “localhost” indicates that the listening port be
195 bound for local use only, while an empty address or ‘*’ indicates
196 that the port should be available from all interfaces.
197 -l login_name
199 Specifies the user to log in as on the remote machine. This also
200 may be specified on a per-host basis in the configuration file.
201 -M Places the ssh client into “master” mode for connection sharing.
203 Multiple -M options places ssh into “master” mode but with con‐
204 firmation required using ssh-askpass(1) before each operation
205 that changes the multiplexing state (e.g. opening a new session).
206 Refer to the description of ControlMaster in ssh_config(5) for
207 details.
208 -m mac_spec
210 A comma-separated list of MAC (message authentication code) algo‐
211 rithms, specified in order of preference. See the MACs keyword
212 for more information.
213 -N Do not execute a remote command. This is useful for just for‐
215 warding ports. Refer to the description of SessionType in
216 ssh_config(5) for details.
217 -n Redirects stdin from /dev/null (actually, prevents reading from
219 stdin). This must be used when ssh is run in the background. A
220 common trick is to use this to run X11 programs on a remote ma‐
221 chine. For example, ssh -n shadows.cs.hut.fi emacs & will start
222 an emacs on shadows.cs.hut.fi, and the X11 connection will be au‐
223 tomatically forwarded over an encrypted channel. The ssh program
224 will be put in the background. (This does not work if ssh needs
225 to ask for a password or passphrase; see also the -f option.)
226 Refer to the description of StdinNull in ssh_config(5) for de‐
227 tails.
228 -O ctl_cmd
230 Control an active connection multiplexing master process. When
231 the -O option is specified, the ctl_cmd argument is interpreted
232 and passed to the master process. Valid commands are: “check”
233 (check that the master process is running), “forward” (request
234 forwardings without command execution), “cancel” (cancel forward‐
235 ings), “exit” (request the master to exit), and “stop” (request
236 the master to stop accepting further multiplexing requests).
237 -o option
239 Can be used to give options in the format used in the configura‐
240 tion file. This is useful for specifying options for which there
241 is no separate command-line flag. For full details of the op‐
242 tions listed below, and their possible values, see ssh_config(5).
243 AddKeysToAgent
245 AddressFamily
246 BatchMode
247 BindAddress
248 BindInterface
249 CanonicalDomains
250 CanonicalizeFallbackLocal
251 CanonicalizeHostname
252 CanonicalizeMaxDots
253 CanonicalizePermittedCNAMEs
254 CASignatureAlgorithms
255 CertificateFile
256 CheckHostIP
257 Ciphers
258 ClearAllForwardings
259 Compression
260 ConnectionAttempts
261 ConnectTimeout
262 ControlMaster
263 ControlPath
264 ControlPersist
265 DynamicForward
266 EnableSSHKeysign
267 EscapeChar
268 ExitOnForwardFailure
269 FingerprintHash
270 ForkAfterAuthentication
271 ForwardAgent
272 ForwardX11
273 ForwardX11Timeout
274 ForwardX11Trusted
275 GatewayPorts
276 GlobalKnownHostsFile
277 GSSAPIAuthentication
278 GSSAPIKeyExchange
279 GSSAPIClientIdentity
280 GSSAPIDelegateCredentials
281 GSSAPIKexAlgorithms
282 GSSAPIRenewalForcesRekey
283 GSSAPIServerIdentity
284 GSSAPITrustDns
285 HashKnownHosts
286 Host
287 HostbasedAcceptedAlgorithms
288 HostbasedAuthentication
289 HostKeyAlgorithms
290 HostKeyAlias
291 Hostname
292 IdentitiesOnly
293 IdentityAgent
294 IdentityFile
295 IgnoreUnknown
296 Include
297 IPQoS
298 KbdInteractiveAuthentication
299 KbdInteractiveDevices
300 KexAlgorithms
301 KnownHostsCommand
302 LocalCommand
303 LocalForward
304 LogLevel
305 LogVerbose
306 MACs
307 Match
308 NoHostAuthenticationForLocalhost
309 NumberOfPasswordPrompts
310 PasswordAuthentication
311 PermitLocalCommand
312 PermitRemoteOpen
313 PKCS11Provider
314 Port
315 PreferredAuthentications
316 ProxyCommand
317 ProxyJump
318 ProxyUseFdpass
319 PubkeyAcceptedAlgorithms
320 PubkeyAuthentication
321 RekeyLimit
322 RemoteCommand
323 RemoteForward
324 RequestTTY
325 RevokedHostKeys
326 SecurityKeyProvider
327 RequiredRSASize
328 SendEnv
329 ServerAliveInterval
330 ServerAliveCountMax
331 SessionType
332 SetEnv
333 StdinNull
334 StreamLocalBindMask
335 StreamLocalBindUnlink
336 StrictHostKeyChecking
337 SyslogFacility
338 TCPKeepAlive
339 Tunnel
340 TunnelDevice
341 UpdateHostKeys
342 User
343 UserKnownHostsFile
344 VerifyHostKeyDNS
345 VisualHostKey
346 XAuthLocation
347 -p port
349 Port to connect to on the remote host. This can be specified on
350 a per-host basis in the configuration file.
351 -Q query_option
353 Queries for the algorithms supported by one of the following fea‐
354 tures: cipher (supported symmetric ciphers), cipher-auth (sup‐
355 ported symmetric ciphers that support authenticated encryption),
356 help (supported query terms for use with the -Q flag), mac (sup‐
357 ported message integrity codes), kex (key exchange algorithms),
358 kex-gss (GSSAPI key exchange algorithms), key (key types),
359 key-cert (certificate key types), key-plain (non-certificate key
360 types), key-sig (all key types and signature algorithms),
361 protocol-version (supported SSH protocol versions), and sig (sup‐
362 ported signature algorithms). Alternatively, any keyword from
363 ssh_config(5) or sshd_config(5) that takes an algorithm list may
364 be used as an alias for the corresponding query_option.
365 -q Quiet mode. Causes most warning and diagnostic messages to be
367 suppressed.
368 -R [bind_address:]port:host:hostport
370 -R [bind_address:]port:local_socket
371 -R remote_socket:host:hostport
372 -R remote_socket:local_socket
373 -R [bind_address:]port
374 Specifies that connections to the given TCP port or Unix socket
375 on the remote (server) host are to be forwarded to the local
376 side.
377 This works by allocating a socket to listen to either a TCP port
379 or to a Unix socket on the remote side. Whenever a connection is
380 made to this port or Unix socket, the connection is forwarded
381 over the secure channel, and a connection is made from the local
382 machine to either an explicit destination specified by host port
383 hostport, or local_socket, or, if no explicit destination was
384 specified, ssh will act as a SOCKS 4/5 proxy and forward connec‐
385 tions to the destinations requested by the remote SOCKS client.
386 Port forwardings can also be specified in the configuration file.
388 Privileged ports can be forwarded only when logging in as root on
389 the remote machine. IPv6 addresses can be specified by enclosing
390 the address in square brackets.
391 By default, TCP listening sockets on the server will be bound to
393 the loopback interface only. This may be overridden by specify‐
394 ing a bind_address. An empty bind_address, or the address ‘*’,
395 indicates that the remote socket should listen on all interfaces.
396 Specifying a remote bind_address will only succeed if the
397 server's GatewayPorts option is enabled (see sshd_config(5)).
398 If the port argument is ‘0’, the listen port will be dynamically
400 allocated on the server and reported to the client at run time.
401 When used together with -O forward the allocated port will be
402 printed to the standard output.
403 -S ctl_path
405 Specifies the location of a control socket for connection shar‐
406 ing, or the string “none” to disable connection sharing. Refer
407 to the description of ControlPath and ControlMaster in
408 ssh_config(5) for details.
409 -s May be used to request invocation of a subsystem on the remote
411 system. Subsystems facilitate the use of SSH as a secure trans‐
412 port for other applications (e.g. sftp(1)). The subsystem is
413 specified as the remote command. Refer to the description of
414 SessionType in ssh_config(5) for details.
415 -T Disable pseudo-terminal allocation.
417 -t Force pseudo-terminal allocation. This can be used to execute
419 arbitrary screen-based programs on a remote machine, which can be
420 very useful, e.g. when implementing menu services. Multiple -t
421 options force tty allocation, even if ssh has no local tty.
422 -V Display the version number and exit.
424 -v Verbose mode. Causes ssh to print debugging messages about its
426 progress. This is helpful in debugging connection, authentica‐
427 tion, and configuration problems. Multiple -v options increase
428 the verbosity. The maximum is 3.
429 -W host:port
431 Requests that standard input and output on the client be for‐
432 warded to host on port over the secure channel. Implies -N, -T,
433 ExitOnForwardFailure and ClearAllForwardings, though these can be
434 overridden in the configuration file or using -o command line op‐
435 tions.
436 -w local_tun[:remote_tun]
438 Requests tunnel device forwarding with the specified tun(4) de‐
439 vices between the client (local_tun) and the server (remote_tun).
440 The devices may be specified by numerical ID or the keyword
442 “any”, which uses the next available tunnel device. If
443 remote_tun is not specified, it defaults to “any”. See also the
444 Tunnel and TunnelDevice directives in ssh_config(5).
445 If the Tunnel directive is unset, it will be set to the default
447 tunnel mode, which is “point-to-point”. If a different Tunnel
448 forwarding mode it desired, then it should be specified before
449 -w.
450 -X Enables X11 forwarding. This can also be specified on a per-host
452 basis in a configuration file.
453 X11 forwarding should be enabled with caution. Users with the
455 ability to bypass file permissions on the remote host (for the
456 user's X authorization database) can access the local X11 display
457 through the forwarded connection. An attacker may then be able
458 to perform activities such as keystroke monitoring.
459 For this reason, X11 forwarding is subjected to X11 SECURITY ex‐
461 tension restrictions by default. Please refer to the ssh -Y op‐
462 tion and the ForwardX11Trusted directive in ssh_config(5) for
463 more information.
464 -x Disables X11 forwarding.
466 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
468 subjected to the X11 SECURITY extension controls.
469 -y Send log information using the syslog(3) system module. By de‐
471 fault this information is sent to stderr.
472 ssh may additionally obtain configuration data from a per-user configura‐
474 tion file and a system-wide configuration file. The file format and con‐
475 figuration options are described in ssh_config(5).
476
478 The OpenSSH SSH client supports SSH protocol 2.
479 The methods available for authentication are: GSSAPI-based authentica‐
481 tion, host-based authentication, public key authentication, keyboard-in‐
482 teractive authentication, and password authentication. Authentication
483 methods are tried in the order specified above, though
484 PreferredAuthentications can be used to change the default order.
485 Host-based authentication works as follows: If the machine the user logs
487 in from is listed in /etc/hosts.equiv or /etc/ssh/shosts.equiv on the re‐
488 mote machine, the user is non-root and the user names are the same on
489 both sides, or if the files ~/.rhosts or ~/.shosts exist in the user's
490 home directory on the remote machine and contain a line containing the
491 name of the client machine and the name of the user on that machine, the
492 user is considered for login. Additionally, the server must be able to
493 verify the client's host key (see the description of
494 /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, below) for login to be
495 permitted. This authentication method closes security holes due to IP
496 spoofing, DNS spoofing, and routing spoofing. [Note to the administra‐
497 tor: /etc/hosts.equiv, ~/.rhosts, and the rlogin/rsh protocol in general,
498 are inherently insecure and should be disabled if security is desired.]
499 Public key authentication works as follows: The scheme is based on pub‐
501 lic-key cryptography, using cryptosystems where encryption and decryption
502 are done using separate keys, and it is unfeasible to derive the decryp‐
503 tion key from the encryption key. The idea is that each user creates a
504 public/private key pair for authentication purposes. The server knows
505 the public key, and only the user knows the private key. ssh implements
506 public key authentication protocol automatically, using one of the DSA,
507 ECDSA, Ed25519 or RSA algorithms. The HISTORY section of ssl(8) contains
508 a brief discussion of the DSA and RSA algorithms.
509 The file ~/.ssh/authorized_keys lists the public keys that are permitted
511 for logging in. When the user logs in, the ssh program tells the server
512 which key pair it would like to use for authentication. The client
513 proves that it has access to the private key and the server checks that
514 the corresponding public key is authorized to accept the account.
515 The server may inform the client of errors that prevented public key au‐
517 thentication from succeeding after authentication completes using a dif‐
518 ferent method. These may be viewed by increasing the LogLevel to DEBUG
519 or higher (e.g. by using the -v flag).
520 The user creates their key pair by running ssh-keygen(1). This stores
522 the private key in ~/.ssh/id_dsa (DSA), ~/.ssh/id_ecdsa (ECDSA),
523 ~/.ssh/id_ecdsa_sk (authenticator-hosted ECDSA), ~/.ssh/id_ed25519
524 (Ed25519), ~/.ssh/id_ed25519_sk (authenticator-hosted Ed25519), or
525 ~/.ssh/id_rsa (RSA) and stores the public key in ~/.ssh/id_dsa.pub (DSA),
526 ~/.ssh/id_ecdsa.pub (ECDSA), ~/.ssh/id_ecdsa_sk.pub (authenticator-hosted
527 ECDSA), ~/.ssh/id_ed25519.pub (Ed25519), ~/.ssh/id_ed25519_sk.pub (au‐
528 thenticator-hosted Ed25519), or ~/.ssh/id_rsa.pub (RSA) in the user's
529 home directory. The user should then copy the public key to
530 ~/.ssh/authorized_keys in their home directory on the remote machine.
531 The authorized_keys file corresponds to the conventional ~/.rhosts file,
532 and has one key per line, though the lines can be very long. After this,
533 the user can log in without giving the password.
534 A variation on public key authentication is available in the form of cer‐
536 tificate authentication: instead of a set of public/private keys, signed
537 certificates are used. This has the advantage that a single trusted cer‐
538 tification authority can be used in place of many public/private keys.
539 See the CERTIFICATES section of ssh-keygen(1) for more information.
540 The most convenient way to use public key or certificate authentication
542 may be with an authentication agent. See ssh-agent(1) and (optionally)
543 the AddKeysToAgent directive in ssh_config(5) for more information.
544 Keyboard-interactive authentication works as follows: The server sends an
546 arbitrary "challenge" text and prompts for a response, possibly multiple
547 times. Examples of keyboard-interactive authentication include BSD Au‐
548 thentication (see login.conf(5)) and PAM (some non-OpenBSD systems).
549 Finally, if other authentication methods fail, ssh prompts the user for a
551 password. The password is sent to the remote host for checking; however,
552 since all communications are encrypted, the password cannot be seen by
553 someone listening on the network.
554 ssh automatically maintains and checks a database containing identifica‐
556 tion for all hosts it has ever been used with. Host keys are stored in
557 ~/.ssh/known_hosts in the user's home directory. Additionally, the file
558 /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any
559 new hosts are automatically added to the user's file. If a host's iden‐
560 tification ever changes, ssh warns about this and disables password au‐
561 thentication to prevent server spoofing or man-in-the-middle attacks,
562 which could otherwise be used to circumvent the encryption. The
563 StrictHostKeyChecking option can be used to control logins to machines
564 whose host key is not known or has changed.
565 When the user's identity has been accepted by the server, the server ei‐
567 ther executes the given command in a non-interactive session or, if no
568 command has been specified, logs into the machine and gives the user a
569 normal shell as an interactive session. All communication with the re‐
570 mote command or shell will be automatically encrypted.
571 If an interactive session is requested ssh by default will only request a
573 pseudo-terminal (pty) for interactive sessions when the client has one.
574 The flags -T and -t can be used to override this behaviour.
575 If a pseudo-terminal has been allocated the user may use the escape char‐
577 acters noted below.
578 If no pseudo-terminal has been allocated, the session is transparent and
580 can be used to reliably transfer binary data. On most systems, setting
581 the escape character to “none” will also make the session transparent
582 even if a tty is used.
583 The session terminates when the command or shell on the remote machine
585 exits and all X11 and TCP connections have been closed.
586
588 When a pseudo-terminal has been requested, ssh supports a number of func‐
589 tions through the use of an escape character.
590 A single tilde character can be sent as ~~ or by following the tilde by a
592 character other than those described below. The escape character must
593 always follow a newline to be interpreted as special. The escape charac‐
594 ter can be changed in configuration files using the EscapeChar configura‐
595 tion directive or on the command line by the -e option.
596 The supported escapes (assuming the default ‘~’) are:
598 ~. Disconnect.
600 ~^Z Background ssh.
602 ~# List forwarded connections.
604 ~& Background ssh at logout when waiting for forwarded connection /
606 X11 sessions to terminate.
607 ~? Display a list of escape characters.
609 ~B Send a BREAK to the remote system (only useful if the peer sup‐
611 ports it).
612 ~C Open command line. Currently this allows the addition of port
614 forwardings using the -L, -R and -D options (see above). It also
615 allows the cancellation of existing port-forwardings with
616 -KL[bind_address:]port for local, -KR[bind_address:]port for re‐
617 mote and -KD[bind_address:]port for dynamic port-forwardings.
618 !command allows the user to execute a local command if the
619 PermitLocalCommand option is enabled in ssh_config(5). Basic
620 help is available, using the -h option.
621 ~R Request rekeying of the connection (only useful if the peer sup‐
623 ports it).
624 ~V Decrease the verbosity (LogLevel) when errors are being written
626 to stderr.
627 ~v Increase the verbosity (LogLevel) when errors are being written
629 to stderr.
630
632 Forwarding of arbitrary TCP connections over a secure channel can be
633 specified either on the command line or in a configuration file. One
634 possible application of TCP forwarding is a secure connection to a mail
635 server; another is going through firewalls.
636 In the example below, we look at encrypting communication for an IRC
638 client, even though the IRC server it connects to does not directly sup‐
639 port encrypted communication. This works as follows: the user connects
640 to the remote host using ssh, specifying the ports to be used to forward
641 the connection. After that it is possible to start the program locally,
642 and ssh will encrypt and forward the connection to the remote server.
643 The following example tunnels an IRC session from the client to an IRC
645 server at “server.example.com”, joining channel “#users”, nickname
646 “pinky”, using the standard IRC port, 6667:
647 $ ssh -f -L 6667:localhost:6667 server.example.com sleep 10
649 $ irc -c '#users' pinky IRC/127.0.0.1
650 The -f option backgrounds ssh and the remote command “sleep 10” is speci‐
652 fied to allow an amount of time (10 seconds, in the example) to start the
653 program which is going to use the tunnel. If no connections are made
654 within the time specified, ssh will exit.
655
657 If the ForwardX11 variable is set to “yes” (or see the description of the
658 -X, -x, and -Y options above) and the user is using X11 (the DISPLAY en‐
659 vironment variable is set), the connection to the X11 display is automat‐
660 ically forwarded to the remote side in such a way that any X11 programs
661 started from the shell (or command) will go through the encrypted chan‐
662 nel, and the connection to the real X server will be made from the local
663 machine. The user should not manually set DISPLAY. Forwarding of X11
664 connections can be configured on the command line or in configuration
665 files.
666 The DISPLAY value set by ssh will point to the server machine, but with a
668 display number greater than zero. This is normal, and happens because
669 ssh creates a “proxy” X server on the server machine for forwarding the
670 connections over the encrypted channel.
671 ssh will also automatically set up Xauthority data on the server machine.
673 For this purpose, it will generate a random authorization cookie, store
674 it in Xauthority on the server, and verify that any forwarded connections
675 carry this cookie and replace it by the real cookie when the connection
676 is opened. The real authentication cookie is never sent to the server
677 machine (and no cookies are sent in the plain).
678 If the ForwardAgent variable is set to “yes” (or see the description of
680 the -A and -a options above) and the user is using an authentication
681 agent, the connection to the agent is automatically forwarded to the re‐
682 mote side.
683
685 When connecting to a server for the first time, a fingerprint of the
686 server's public key is presented to the user (unless the option
687 StrictHostKeyChecking has been disabled). Fingerprints can be determined
688 using ssh-keygen(1):
689 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
691 If the fingerprint is already known, it can be matched and the key can be
693 accepted or rejected. If only legacy (MD5) fingerprints for the server
694 are available, the ssh-keygen(1) -E option may be used to downgrade the
695 fingerprint algorithm to match.
696 Because of the difficulty of comparing host keys just by looking at fin‐
698 gerprint strings, there is also support to compare host keys visually,
699 using random art. By setting the VisualHostKey option to “yes”, a small
700 ASCII graphic gets displayed on every login to a server, no matter if the
701 session itself is interactive or not. By learning the pattern a known
702 server produces, a user can easily find out that the host key has changed
703 when a completely different pattern is displayed. Because these patterns
704 are not unambiguous however, a pattern that looks similar to the pattern
705 remembered only gives a good probability that the host key is the same,
706 not guaranteed proof.
707 To get a listing of the fingerprints along with their random art for all
709 known hosts, the following command line can be used:
710 $ ssh-keygen -lv -f ~/.ssh/known_hosts
712 If the fingerprint is unknown, an alternative method of verification is
714 available: SSH fingerprints verified by DNS. An additional resource
715 record (RR), SSHFP, is added to a zonefile and the connecting client is
716 able to match the fingerprint with that of the key presented.
717 In this example, we are connecting a client to a server,
719 “host.example.com”. The SSHFP resource records should first be added to
720 the zonefile for host.example.com:
721 $ ssh-keygen -r host.example.com.
723 The output lines will have to be added to the zonefile. To check that
725 the zone is answering fingerprint queries:
726 $ dig -t SSHFP host.example.com
728 Finally the client connects:
730 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
732 [...]
733 Matching host key fingerprint found in DNS.
734 Are you sure you want to continue connecting (yes/no)?
735 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
737
739 ssh contains support for Virtual Private Network (VPN) tunnelling using
740 the tun(4) network pseudo-device, allowing two networks to be joined se‐
741 curely. The sshd_config(5) configuration option PermitTunnel controls
742 whether the server supports this, and at what level (layer 2 or 3 traf‐
743 fic).
744 The following example would connect client network 10.0.50.0/24 with re‐
746 mote network 10.0.99.0/24 using a point-to-point connection from 10.1.1.1
747 to 10.1.1.2, provided that the SSH server running on the gateway to the
748 remote network, at 192.168.1.15, allows it.
749 On the client:
751 # ssh -f -w 0:1 192.168.1.15 true
753 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
754 # route add 10.0.99.0/24 10.1.1.2
755 On the server:
757 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
759 # route add 10.0.50.0/24 10.1.1.1
760 Client access may be more finely tuned via the /root/.ssh/authorized_keys
762 file (see below) and the PermitRootLogin server option. The following
763 entry would permit connections on tun(4) device 1 from user “jane” and on
764 tun device 2 from user “john”, if PermitRootLogin is set to
765 “forced-commands-only”:
766 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
768 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
769 Since an SSH-based setup entails a fair amount of overhead, it may be
771 more suited to temporary setups, such as for wireless VPNs. More perma‐
772 nent VPNs are better provided by tools such as ipsecctl(8) and
773 isakmpd(8).
774
776 ssh will normally set the following environment variables:
777 DISPLAY The DISPLAY variable indicates the location of the
779 X11 server. It is automatically set by ssh to
780 point to a value of the form “hostname:n”, where
781 “hostname” indicates the host where the shell runs,
782 and ‘n’ is an integer ≥ 1. ssh uses this special
783 value to forward X11 connections over the secure
784 channel. The user should normally not set DISPLAY
785 explicitly, as that will render the X11 connection
786 insecure (and will require the user to manually
787 copy any required authorization cookies).
788 HOME Set to the path of the user's home directory.
790 LOGNAME Synonym for USER; set for compatibility with sys‐
792 tems that use this variable.
793 MAIL Set to the path of the user's mailbox.
795 PATH Set to the default PATH, as specified when compil‐
797 ing ssh.
798 SSH_ASKPASS If ssh needs a passphrase, it will read the
800 passphrase from the current terminal if it was run
801 from a terminal. If ssh does not have a terminal
802 associated with it but DISPLAY and SSH_ASKPASS are
803 set, it will execute the program specified by
804 SSH_ASKPASS and open an X11 window to read the
805 passphrase. This is particularly useful when call‐
806 ing ssh from a .xsession or related script. (Note
807 that on some machines it may be necessary to redi‐
808 rect the input from /dev/null to make this work.)
809 SSH_ASKPASS_REQUIRE Allows further control over the use of an askpass
811 program. If this variable is set to “never” then
812 ssh will never attempt to use one. If it is set to
813 “prefer”, then ssh will prefer to use the askpass
814 program instead of the TTY when requesting pass‐
815 words. Finally, if the variable is set to “force”,
816 then the askpass program will be used for all
817 passphrase input regardless of whether DISPLAY is
818 set.
819 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to
821 communicate with the agent.
822 SSH_CONNECTION Identifies the client and server ends of the con‐
824 nection. The variable contains four space-sepa‐
825 rated values: client IP address, client port num‐
826 ber, server IP address, and server port number.
827 SSH_ORIGINAL_COMMAND This variable contains the original command line if
829 a forced command is executed. It can be used to
830 extract the original arguments.
831 SSH_TTY This is set to the name of the tty (path to the de‐
833 vice) associated with the current shell or command.
834 If the current session has no tty, this variable is
835 not set.
836 SSH_TUNNEL Optionally set by sshd(8) to contain the interface
838 names assigned if tunnel forwarding was requested
839 by the client.
840 SSH_USER_AUTH Optionally set by sshd(8), this variable may con‐
842 tain a pathname to a file that lists the authenti‐
843 cation methods successfully used when the session
844 was established, including any public keys that
845 were used.
846 TZ This variable is set to indicate the present time
848 zone if it was set when the daemon was started
849 (i.e. the daemon passes the value on to new connec‐
850 tions).
851 USER Set to the name of the user logging in.
853 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
855 “VARNAME=value” to the environment if the file exists and users are al‐
856 lowed to change their environment. For more information, see the
857 PermitUserEnvironment option in sshd_config(5).
858
860 ~/.rhosts
861 This file is used for host-based authentication (see above). On
862 some machines this file may need to be world-readable if the
863 user's home directory is on an NFS partition, because sshd(8)
864 reads it as root. Additionally, this file must be owned by the
865 user, and must not have write permissions for anyone else. The
866 recommended permission for most machines is read/write for the
867 user, and not accessible by others.
868 ~/.shosts
870 This file is used in exactly the same way as .rhosts, but allows
871 host-based authentication without permitting login with
872 rlogin/rsh.
873 ~/.ssh/
875 This directory is the default location for all user-specific con‐
876 figuration and authentication information. There is no general
877 requirement to keep the entire contents of this directory secret,
878 but the recommended permissions are read/write/execute for the
879 user, and not accessible by others.
880 ~/.ssh/authorized_keys
882 Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used
883 for logging in as this user. The format of this file is de‐
884 scribed in the sshd(8) manual page. This file is not highly sen‐
885 sitive, but the recommended permissions are read/write for the
886 user, and not accessible by others.
887 ~/.ssh/config
889 This is the per-user configuration file. The file format and
890 configuration options are described in ssh_config(5). Because of
891 the potential for abuse, this file must have strict permissions:
892 read/write for the user, and not writable by others.
893 ~/.ssh/environment
895 Contains additional definitions for environment variables; see
896 ENVIRONMENT, above.
897 ~/.ssh/id_dsa
899 ~/.ssh/id_ecdsa
900 ~/.ssh/id_ecdsa_sk
901 ~/.ssh/id_ed25519
902 ~/.ssh/id_ed25519_sk
903 ~/.ssh/id_rsa
904 Contains the private key for authentication. These files contain
905 sensitive data and should be readable by the user but not acces‐
906 sible by others (read/write/execute). ssh will simply ignore a
907 private key file if it is accessible by others. It is possible
908 to specify a passphrase when generating the key which will be
909 used to encrypt the sensitive part of this file using AES-128.
910 ~/.ssh/id_dsa.pub
912 ~/.ssh/id_ecdsa.pub
913 ~/.ssh/id_ecdsa_sk.pub
914 ~/.ssh/id_ed25519.pub
915 ~/.ssh/id_ed25519_sk.pub
916 ~/.ssh/id_rsa.pub
917 Contains the public key for authentication. These files are not
918 sensitive and can (but need not) be readable by anyone.
919 ~/.ssh/known_hosts
921 Contains a list of host keys for all hosts the user has logged
922 into that are not already in the systemwide list of known host
923 keys. See sshd(8) for further details of the format of this
924 file.
925 ~/.ssh/rc
927 Commands in this file are executed by ssh when the user logs in,
928 just before the user's shell (or command) is started. See the
929 sshd(8) manual page for more information.
930 /etc/hosts.equiv
932 This file is for host-based authentication (see above). It
933 should only be writable by root.
934 /etc/ssh/shosts.equiv
936 This file is used in exactly the same way as hosts.equiv, but al‐
937 lows host-based authentication without permitting login with
938 rlogin/rsh.
939 /etc/ssh/ssh_config
941 Systemwide configuration file. The file format and configuration
942 options are described in ssh_config(5).
943 /etc/ssh/ssh_host_key
945 /etc/ssh/ssh_host_dsa_key
946 /etc/ssh/ssh_host_ecdsa_key
947 /etc/ssh/ssh_host_ed25519_key
948 /etc/ssh/ssh_host_rsa_key
949 These files contain the private parts of the host keys and are
950 used for host-based authentication.
951 /etc/ssh/ssh_known_hosts
953 Systemwide list of known host keys. This file should be prepared
954 by the system administrator to contain the public host keys of
955 all machines in the organization. It should be world-readable.
956 See sshd(8) for further details of the format of this file.
957 /etc/ssh/sshrc
959 Commands in this file are executed by ssh when the user logs in,
960 just before the user's shell (or command) is started. See the
961 sshd(8) manual page for more information.
962
964 ssh exits with the exit status of the remote command or with 255 if an
965 error occurred.
966
968 IPv6 address can be used everywhere where IPv4 address. In all entries
969 must be the IPv6 address enclosed in square brackets. Note: The square
970 brackets are metacharacters for the shell and must be escaped in shell.
971
973 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
974 tun(4), ssh_config(5), ssh-keysign(8), sshd(8)
975
977 S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned
978 Numbers, RFC 4250, January 2006.
979 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture,
981 RFC 4251, January 2006.
982 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol,
984 RFC 4252, January 2006.
985 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer
987 Protocol, RFC 4253, January 2006.
988 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC
990 4254, January 2006.
991 J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell
993 (SSH) Key Fingerprints, RFC 4255, January 2006.
994 F. Cusack and M. Forssen, Generic Message Exchange Authentication for the
996 Secure Shell Protocol (SSH), RFC 4256, January 2006.
997 J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break
999 Extension, RFC 4335, January 2006.
1000 M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport
1002 Layer Encryption Modes, RFC 4344, January 2006.
1003 B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport
1005 Layer Protocol, RFC 4345, January 2006.
1006 M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for
1008 the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
1009 J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File
1011 Format, RFC 4716, November 2006.
1012 D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the
1014 Secure Shell Transport Layer, RFC 5656, December 2009.
1015 A. Perrig and D. Song, Hash Visualization: a New Technique to improve
1017 Real-World Security, 1999, International Workshop on Cryptographic
1018 Techniques and E-Commerce (CrypTEC '99).
1019
1021 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
1022 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
1023 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
1024 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
1025 versions 1.5 and 2.0.
1026BSD September 10, 2021 BSD